In plasma processing systems, semiconductor wafers are placed on static substrate holders comprising a planar or substantially planar electrode, such as electrostatic chucks (ESCs) or chucks provided with mechanical wafer clamps, where the substrates, or wafers, are exposed to ions from the plasma formed using radio frequency (RF) energy. Energy of the ions incident on the wafers from the plasma is typically independently controlled by varying the RF bias power level, while the ion density is usually controlled by control of an independent plasma source. The plasma source can be, for example, an inductively coupled plasma (ICP) source, a dual capacitively-coupled plasma (CCP) source having a secondary top electrode biased at another RF frequency, a helicon plasma source, a microwave plasma source, a magnetron, or some other independently operated source of plasma energy.
An RF biased ESC operates as CCP source in which a wafer having a surface to be etched or coated is placed on a wafer support electrode. The wafer support electrode is typically coupled to an RF generator through a blocking capacitor and an impedance matching unit, which does not allow real current to flow from the electrode to the RF generator. FIG. 1A is a diagram that represents a bulk plasma 10 in a plasma process chamber in which a substrate 15 is supported on a substrate support 14 for processing. A plasma sheath 12 occupies a boundary between the bulk plasma 10 and the surface of the wafer 15. Between the bulk plasma 10 and the plasma sheath 12 is a region that can be referred to as the plasma presheath 13.
At the plasma-to-surface boundary, electrons escape from the plasma and charge the wafer surface with a negative charge. This occurs because the plasma and the biased ESC electrode will find a condition at which electron current reaching the electrode from the plasma exactly balances ion current averaged over one RF cycle. Since electrons are more mobile than ions, the electrode acquires a negative potential that will limit electron current and encourage positive ion current from the plasma to the wafer surface. This negative potential is called the self-bias voltage and results in an energetic ion bombardment of the surface from the plasma, which can, for example, etch the wafer surface. Since the wafer is not formed of a conductive material, and since the substrate holder is usually coated with a dielectric or insulating material, the actual self-bias on the conductive portion of holder closer to the RF source is more negative than the self-bias on the top of the support. Changing applied RF bias power can control the ion bombardment energy and can partially affect the ion flux as well.
The initial kinetic energy of the ions in bulk plasma 10 is related to the gas temperature and, typically, is within a range of 0.05 to 0.1 electron volts (eV). Ion motion in the plasma 10 is random due to collisions with background gas atoms or molecules. Also, a directional component may be present in the plasma 10 due to ambipolar diffusion of the ions in an ambipolar electric field inside the plasma 10, which is usually small, approximately 0.1 to 1 V/cm, but which can contribute to asymmetry in feature processing by bombarding ions. Before ions enter the sheath 12 from the plasma 10, they gain some directional energy within the presheath 13. This energy is approximately half the electron temperature (Te) of the plasma 10. Sheath theory tells us that ions enter plasma sheaths with a so-called Bohm velocity, which is proportional to kTe/mi, where mi is the mass of the ion. In an actual plasma sheath 12, ions are accelerated by a sheath voltage Esheath that is the difference between the plasma voltage and the bias voltage on the wafer 15. The sheath voltage Esheath is much larger than the energy gain in the presheath 13, and is much-much larger than the kinetic energy of the ions in the bulk plasma 10.
Due to the combined random kinetic energy gained in the bulk plasma 10 and directional energy gained within the presheath 13 and sheath 12, ions will hit the surface of the wafer 15 with a particular angle, or in a particular distribution of angles. This distribution of angles can be referred to as the Ion Angular Distribution Function (IADF) 16, which can be defined for given process conditions. In typical systems of the prior art, most of the ions are within a small angle cone 17 of less than about 20 degrees when pressures are in the range of 1 milli-Torr (mTorr) to several 100 s of mTorr.
Experimental data from literature show the IADF at the wafer 15 in an ICP plasma process to be approximately as illustrated as curve 18 in FIG. 1B, shown for pressures at about 2.5 mTorr, to that as illustrated as curve 19 at pressures at about 50 mTorr. The graphs are plots of the angular distribution of ions incident at a given point on the surface of a wafer, with the angle being measured relative to a vector normal to the wafer surface at the given point. The graph of FIG. 1B shows the distribution at angles measured in a vertical plane through the given point. This distribution is not necessarily the same in every direction or in every vertical plane through the point, and may therefore be regarded as a three-dimensional function of an angle φ to the normal, through a plane oriented at some angle θ to a reference line on the wafer's surface.
Furthermore, the IADF is not necessarily the same at every point on the surface of the wafer, and can vary across the surface of the wafer. Therefore, the IADF can also be considered as a function of φ and θ, that additionally varies as a function of the x,y coordinates on the wafer surface, or more conveniently, of the R,⊖ coordinates of the points on the wafer surface. It is not uncommon to experience different ion incidence angles between the center and the edge of a wafer, for example.
The IADF plays an important role in feature coverage in ionized physical vapor deposition (iPVD) and other ion-controlled deposition and etching processes. Control of IADF to produce ideal feature coverage has been a challenge because a lack of independent process variables that allow direct control of the IADF at the wafer surface that is in contact with the plasma. Movement of the substrate using a substrate holder comprising a planar or substantially planar electrode to modify the IADF may negatively affect an electromagnetic field of the plasma. Additionally, movement of the substrate to modify an IADF may provide an opportunity to damage or otherwise compromise the integrity of the substrate.